The conventional incadescent lamp used for home lighting is commonly known as the A-line gas filled lamp having a wattage between about 25 and 150 watts. The construction parameters for such lamps are substantially uniform among different manufacturers, the lamps being designed for operation from a nominal voltage of 120 volts, 60 cycle AC with standard power ratings of 40, 60, 75, 100 and 150 watts. Such lamps are rated for average operating life, and for standard lamps a rated life of 1,000 hours or less is typical. For convenience, these may be termed short-life lamps as compared with known lamps having extended life ranging upward from 1,000 hours to several years. The primary cause of lamp failure is cohesive failure of the tungsten filament, said filament failure generally being caused by evaporation of tungsten, said evaporation being a temperature-dependent phenomenon.
One way of increasing the life of a given lamp is by operating the tungsten filament at a reduced temperature, as may be achieved by changing the design of the filament. A typical 100 watt lamp has a coiled tungsten wire filament of 0.0635 mm diameter and an uncoiled length of 577 mm. It has an operating temperature of 2577 degrees C. and an average operating life of 750 hours. The life of this lamp could be increased while retaining the same wattage rating by changing the filament configuration so that it operates at a lower temperature; however, the efficiency in terms of lumens per watt would be decreased.
Another way of extending the operating life of an incandescent lamp is to reduce the effective voltage across the filament. This has been done in the prior art by connecting a rectifying diode in series with the filament so that only alternate half cycles of the supply voltage are applied to the filament. This has the effect of reducing the time average value of voltage to about 0.7 of that produced by the unrectified supply voltage. Consequently, the filament is operated at a substantially lower temperature and also at a lower efficiency in terms of lumens per watt. The use of a rectifying diode installed in a socket for a lamp to prolong lamp life is disclosed in U.S. Pat. Nos.: 3,450,893; and 3,963,956. Lamps which incorporate the diode into the structure of the lamp are disclosed in U.S. Pat. Nos.: 3,148,305; 3,869,631; and 4,480,212.
One difficulty encountered in the use of a diode for extending lamp life is that the reduction of effective voltage without a change in the filament design results in a large decrease in filament temperature, and the light output of the lamp is substantially diminished. Operation at the reduced temperature also results in lower efficiency of the filament. For example, the standard 100 watt lamp mentioned above has an operating temperature of 2577 degrees C. without a diode and the light output is 1750 lumens. With a diode in this lamp in series with the filament, the operating temperature is reduced to 2164 degrees C. and the light output is 510 lumens. The diode has the effect of reducing power consumption of the lamp from 100 watts to 58.8 watts. The average life for the lamp without the diode is 750 hours and the same lamp with a diode is 8.6 years.
The above-cited U.S. Pat. No. 4,480,212 recognizes the desirability of shielding the diode from the heat of the filament, and employs toward such objective a reflector within the glass bulb. The positioning of the diode externally to the lamp, namely upon the bottom of the threaded base of the lamp is disclosed in U.S. Pat. Nos. 3,148,305 and 3,823,339. Although such positioning of the diode causes it to be cooler than when enclosed within the glass bulb, further improvement is desirable.
A general object of this invention is to overcome certain disadvantages of the prior art to provide an improved incandescent lamp having an extended operating life and increased efficiency.
It is a further object of the present invention to provide an improved incadescent lamp as in the foregoing object which does not require a specialized filament or additional components within the glass envelope component of the lamp.
These objects and other objects and advantages of the invention will be apparent from the following description.